Because of their excellent corrosion resistance, titanium plates are used as the starting material for heat exchangers in various plants such as chemical plants, power plants and food processing plants. For a plate-type heat exchanger, among others, it is intended to raise the heat exchanging efficiency by increasing the surface area of a titanium sheet by forming recesses and protrusions in the sheet by press forming, which requires excellent formability.
Patent Document 1 discloses a technique including: forming an oxide film and a nitride film by heating in an oxidizing atmosphere or a nitriding atmosphere; thereafter performing bending or pulling out to introduce fine cracks into these films and to expose the titanium metal; and thereafter scarfing in an acid aqueous solution in which titanium metal is soluble to form dense and deep irregularities. Patent Document 1 discloses that the oil retainability of a lubricating oil increases and the lubricity improves, and that by causing an oxide film and a nitride film to remain on the surface or by the formation thereof, the lubricity further improves.
Patent Document 2 discloses that, by performing pickling and skin pass rolling after atmospheric annealing to thereby make a surface roughness Ra, a maximum height Rz and a degree of strain (Rsk) fall within a specific numerical value range, oil retainability can be exerted and the inducement of cracks caused by the notch effect can be prevented, and the formability improves. Further, by making the Vickers hardness at a measurement load of 0.098 N at the surface higher than a Vickers hardness at a measurement load of 4.9 N and making the difference therebetween not more than 45, the occurrence of surface cracks during forming is prevented.
Patent Document 3 discloses a titanium plate in which the arithmetic average roughness of the surface in a direction parallel to the rolling direction is not less than 0.25 μm and not more than 2.5 μm, and the Vickers hardness at a test load of 0.098 N at the surface is 20 or more higher than the Vickers hardness at a test load of 4.9 N, and the Vickers hardness at the test load of 4.9 N is not more than 180. The Patent Document 3 discloses that, by making the roughness of the surface of the titanium plate coarse to a certain extent, the amount of lubricant that is drawn-in between the titanium plate and the forming press tooling during press forming is increased, and the formability improves.
Patent Document 4 discloses that, by chemically or mechanically removing a region of 0.2 μm from the surface, the surface hardness at a load of 200 gf (1.96 N) is made 170 or less and the thickness of an oxide film is made 150 Å or more by eliminating, during cold working, residual oil which was scored into the surface and thereafter performing vacuum annealing. The Patent Document 4 discloses that, by this means, without impairing the formability of the starting material, the lubricity with respect to the die and tooling during forming is maintained, and the formability improves.